1. Field of the Invention
The invention relates to optical communication equipment and, more specifically, to micro-electromechanical devices for use in such equipment.
2. Description of the Related Art
Optical communication equipment often employs micro-electromechanical systems (MEMS). A typical MEMS system may include an array of micro-machined mirrors, each mirror individually movable in response to an electrical signal. Such an array may be employed in an optical cross-connect, in which each mirror in the array receives a beam of light, for example, from an input optical fiber. The beam is reflected from the mirror and can be redirected to a different location, e.g., at which is located an output optical fiber, by rotating the mirror. More details on the principle of operation and methods of manufacture of MEMS devices including mirror arrays may be found, for example, in commonly assigned U.S. Pat. No. 6,201,631, the teachings of which are incorporated herein by reference.
One problem with prior art MEMS devices is referred to as xe2x80x9csnap-down.xe2x80x9d More specifically, when the voltage applied to an actuating electrode in such device approaches a critical value, the tilt angle of the mirror begins to increase rapidly and nonlinearly with the voltage. This behavior may cause a collision of the mirror against the electrode and/or wafer, damaging the mirror and rendering the MEMS device inoperable.
The problems in the prior art are addressed, in accordance with the principles of the invention, by a control circuit that controls the voltages applied to the one or more actuating electrodes of a MEMS device. The control circuit receives a sensor signal, e.g., from a position sensor corresponding to a MEMS device, indicative of the current position of a movable part of the MEMS device relative to a stationary part of the MEMS device. Based on the sensor signal, the control circuit generates a control signal for the MEMS device actuator to achieve a desired orientation of the movable part with respect to the stationary part. The control circuit may include a variable gain amplifier whose output depends on the sensor signal and a reference signal corresponding to a desired equilibrium position for the movable part. The desired equilibrium angle can be changed by adjusting the reference signal applied to the amplifier.
In a MEMS device in which the movable part is a mirror that is rotatably coupled to the stationary part, the control circuit can stabilize the mirror at relatively large tilt angles and, as a result, extend the available angular range beyond the snap-down angle. Since the number of MEMS device mirrors that can be arrayed in an optical cross-connect is a function of the available angular range, in accordance with the principles of the invention, the number of channels in a cross-connect may be substantially increased.